In the field of magnetic recording using heads and media, there is a demand for further improvement in the performance of magnetic recording media and magnetic heads in association with the high recording density of a magnetic disk device.
The magnetic recording medium is a discontinuous medium where magnetic grains aggregate, and where each magnetic grain has a single magnetic domain structure. In the magnetic recording medium, one recording bit is composed of a plurality of magnetic grains. Consequently, in order to enhance the recording density, unevenness at a boundary of adjacent recording bits must be diminished by reducing the size of the magnetic grains. However, if the size of the magnetic grains is reduced, the thermal stability of magnetization of the magnetic grains is reduced, in association with the reduction of volume of the magnetic grains.
As a countermeasure against this problem, an increase of magnetic anisotropy energy Ku in the magnetic grains can be considered, but the increase of Ku causes an increase of an anisotropic magnetic field (coercive force) of the magnetic recording medium. In the meantime, the upper limit of the recording magnetic field intensity by the magnetic head is primarily determined according to saturation magnetic flux density of a soft magnetic material that configures a magnetic core within the head. Consequently, if the anisotropic magnetic field of the magnetic recording medium exceeds a tolerance value, which is determined from the upper limit of the recording magnetic field intensity, it becomes impossible to record into a magnetic recording medium.
At present, one method for solving the problem of thermal stability is energy assisted recording in which a magnetic recording medium formed with a magnetic material with high Ku is used. In this method, it is proposed to provide supplemental energy to the medium when recording to decrease the effective recording magnetic field intensity. The recording method using a microwave magnetic field as the supplemental energy source is referred to as microwave assisted magnetic recording (MAMR), and research and development are in progress for practical uses.
In the microwave assisted magnetic recording, the application of the microwave magnetic field in the medium in-plane direction of a frequency according to an effective magnetic field (Heff) relating to magnetization of a recording layer in the magnetic recording medium excites precession movement of the magnetization of the recording layer, and recording capability of a magnetic head is assisted.
As one example of a magnetic head using a microwave assisted magnetic recording method, as shown in FIG. 15, a magnetic head is proposed that is equipped with a main magnetic pole layer 6′ that generates a recording magnetic field for application to a magnetic recording medium; a wraparound shield composed of a trailing shield 81′ and side shields 82′ and 83′; and a spin torque oscillator (STO) 10′ that is disposed in the write gap between the main magnetic pole layer 6′ and the trailing shield 81′, and that has a multilayer structure with magnetic thin films (for example, U.S. Pat. No. 8,320,079). The STO 10′ generates a microwave magnetic field in the in-plane direction by its self-oscillation. Precession movement of the magnetization of the recording layer is excited by applying the microwave magnetic field to the magnetic recording medium, and magnetization reversal in the perpendicular direction in the recording layer is assisted.
In such a magnetic head, in order to provide a sufficient assist effect by the STO 10′, it is necessary to improve the oscillation frequency of the STO 10′. Consequently, it is desired to narrow the write gap as much as possible, and increase the magnetic intensity applied to the STO 10′. However, there is a limitation to narrowing the write gap where the STO 10′ is formed, and it is difficult to increase the magnetic field intensity applied to the STO 10′.
Further, in the magnetic recording device using the magnetic head, to improve recording density it is necessary to improve both bit per inch (BPI) and track per inch (TPI). For the purpose of improvement of BPI and TPI, it is required to steepen the magnetic field gradient of the recording magnetic field in the down track direction and the cross track direction, respectively. If the track width where signals are recorded is narrowed in order to accomplish high recording density, record(s) in adjacent track(s) may be erased, which should be prevented by steepening the recording magnetic field gradient in the cross track direction.